docs: def sorted_groupby(df, groupby):
      """
      Perform a groupby on a DataFrame using a specific column
      and assuming that that column is sorted.
  
      Parameters
      ----------
      df : pandas.DataFrame
      groupby : object
          Column name on which to groupby. This column must be sorted.
  
      Returns
      -------
      generator
          Yields pairs of group_name, DataFrame.
  
      """
      start = 0
      prev = df[groupby].iloc[start]
      for i, x in enumerate(df[groupby]):
          if x != prev:
              yield prev, df.iloc[start:i]
              prev = x
              start = i
      # need to send back the last group
      yield prev, df.iloc[start:]